1. Field of the Invention
This invention relates to ultra wideband (UWB) precision geolocation. More particularly, the invention relates equipment and methods for attaining reliable transmission, reception and processing of ultra wideband electromagnetic pulses, over distances as great as several kilometers, to determine precise (centimeter-accurate) position measurements of a mobile platform.
2. Background of Related Art
Ultra wideband waveforms have been used to achieve extremely fine, centimeter-type resolutions because of their extremely short (subnanosecond to nanosecond) durations. With the advent of very high speed, high sensitivity detectors (see, for example, U.S. Pat. No. 5,901,172 to Fontana and Larrick, entitled ULTRA WIDEBAND RECEIVER WITH HIGH SPEED NOISE AND INTERFERENCE TRACKING THRESHOLD, incorporated herein by reference), the feasibility of such systems has been demonstrated. Applications of UWB-based radar and time domain reflectometry have included such diverse fields as high resolution radar altimetry, liquid level sensing, collision and obstacle avoidance, etc.
With the ability to achieve range resolutions at centimeter levels (time-of-flight measurements to better than a few tens of picoseconds), UWB can also be used to provide a precise geolocation capability similar to that achieved with real time kinematic (RTK) GPS solutions. In the absence of adequate satellite coverage (e.g., operation under jungle canopies, near various RF obstructions such as mine walls, non-ideal solar flux levels, areas of poor satellite visibility, etc.), such a wireless UWB solution can augment or even replace conventional RTK-based GPS systems. In the complete absence of satellite coverage due to military shut down during war time or satellite failures, the need for a remote system becomes even more compelling.
In GPS-based rapid-static and kinematic positioning systems, measurement ambiguity is resolved by converting ambiguous carrier-phase measurements into unambiguous ranges. With the UWB-based positioning system described here, ambiguous arrival time-differences are converted into unambiguous ranges. This may be described as the time-domain "dual" of the carrier-phase approach, and permits operation under a wide variety of circumstances for which GPS coverage is either unavailable or seriously degraded.
In the prior art, short pulse (impulse), baseband or ultra wideband (UWB) waveforms have been proposed to determine the relative position of a platform or target for such applications as ship docking (Ross, U.S. Pat. No. 4,510,496), precision radar ranging for intrusion detection and alert systems (Woolfolk, U.S. Pat. No. 5,148,175), speed and motion sensors (Mara, Nicolson and Ross, U.S. Pat. No. 4,152,701; McEwan, U.S. Pat. No, 5,361,070); target range detection of slowly moving targets (Henning and Woolfolk, U.S. Pat. No. 5,049,887), liquid level sensing (Rama and Robbins, U.S. Pat. No. 4,489,601), and others. In each of these patents, only the relative position of the target with respect to a fixed position was of interest, with distance to the target being the primary measurement to be performed.
In addition, Japanese Application No. 5-223916 describes a sonar-based position measuring system for underwater use. Japanese Patent Application No. 61-14584 discloses an RF system for measuring the remote position of an object using plural spatially located transmitters and receivers. Japanese Application No. 64-31077 describes a position measurement system using time-of-arrival measurements and an independent synchronizing source wherein the transmitted signals apparently comprise conventional RF emissions. UK Patent Application GB 2254508A also discloses a location determination system that employs time-of-arrival computations to determine position.
However, there has also been interest in the use of ultra wideband signals for the determination of the absolute position of an object:
For example, in McEwan (U.S. Pat. No. 5,510,800), a very short range (&lt;10 ft.) time-of-flight radio location system was described. In the suggested implementation, a single UWB transmitter is used with a multiplicity of UWB receivers to determine a set of time-of-flight measurements from which absolute positional information can be computed provided the absolute locations of each UWB receiver can be measured a priori. Timing information for McEwan's UWB positioning system was derived locally and distributed electrically (via cables) to the transmitter and associated receiver units. The primary limitation of such a system is the need for accurate clock distribution with tightly controlled skew (i.e., the difference in times of arrival between pulses sent along different cables). Thus, such a system necessitates the use of a set of precision cables (or, as a minimum, a set of cables having known or accurately measured group delays). As a consequence, as pointed out by McEwan, the applicability of such an approach is limited to very short distances for which the time-of-flight measurements are less than ten nanoseconds (i.e., range less than ten feet).
In McEwan (U.S. Pat. No. 5,589,938), another short range radio locator system is described in which a single transmitter and multiplicity of receivers is again used to obtain precision absolute positioning information. In this approach, however, one of the receivers is used to electrically provide a synchronizing gating pulse to itself and to the other receivers. The data output from each receiver is, in turn, sent to the synchronizing receiver's processor (CPU) with which the absolute position of the roving transmitter is calculated. In this case, in addition to a set of carefully measured or calibrated cables (for precision clock distribution), additional cabling must be provided between the receivers for the relay of measured time-of-flight data for position computation. Unlike the system of U.S. Pat. No. 5,510,800, McEwan adopts an untethered transmitter, but still utilizes a set of interconnected, cabled receiver units. Such an approach thus limits the achievable range over which the system can effectively (and economically) operate, e.g., to only a few meters.
For many applications of practical interest, it is essential that all equipment be untethered; i.e., no physical wires or cables between transmitters, receivers, or to a central processor or CPU. In this fashion, the range over which precision geolocation can be achieved is limited only by the ranges over which reliable communications can be established between units. For ultra wideband detectors with sensitivities as described in incorporated U.S. application Ser. No. 08/872,729 mentioned above, these ranges can be several kilometers, permitting the use of such a system for the augmentation or replacement of more conventional GPS RTK precision geolocation systems.
In addition, to achieve untethered operation over distances of several hundred meters to kilometers, it is essential that wireless transmissions be used to allow individual units to be precisely time synchronized. This obviates the need for costly cables and the associated high costs of installation and maintenance.